Granular ammonium nitrate explosive



Patented Apr. 30, 1940 mm STATES PATENT ()FFICE GRANULAR AMMONIUMNITRATE EXPLO SIVE Melvin A. Cook, Donald B. Gawthrop, and Milton H.Wahl, Woodbury, N. 3., and Clifiord A.

Woodbury, Media,

assignors to E. ll.

du Pont de Ncmours & Company, Wilmington, DeL, a corporation of DelawareNo Drawing. Original application April 21, 1939,

Serial No. 269,102. Divided and this application July 28, 1939, SerialNo. 287,052

6 Claims.

' essential when the explosive is to be used in the presence ofinflammable mine gases. The permissible explosives are at a disadvantagein one respect, however, when compared with black blasting powder, adeflagrating explosive, in that they do not exert the characteristicsustained heaving action that results with the latter explosive.Consequently, difliculties are met when using permissibles with certaintypes of coal in obtaining a satisfactory grade of lump coal. A markedadvance in the art would be effected if an explosive could be designedwhich would possess the blasting characteristics of black powder whileat the same time representing no backward step with respect to thesafety features of the best of the permissibles.

The object of the present invention is an improved explosive compositionespecially adapted to the mining of coal, said composition having animproved blasting action in this respect over conventional coal miningexplosives. A further object is an improved detonating explosive whichexhibits a sustained heaving action as contrasted with the shatteringaction characteristic of explosives adapted to quarry operations, saidexplosive being characterized by its ability to produce greater yieldsof lump coal. Another object is an improved detonating explosive whichpossesses the blasting characteristics of black powder, yet exhibits thesafety features of the permissibles. Additional objects will bedisclosed as the invention is described more fully hereinafter.

These objects are achieved and the foregoing disadvantages overcome bybinding together particles of ammonium nitrate explosive compositioninto sized aggregates by means of a binding agent substantiallyinsoluble in liquid explosive nitric esters, and coating said sizedaggregates with a liquid explosive nitric ester in an amount notexceeding 10% by weight. One preferred product is in the form of grainsof such size that substantially all are held on a 30-mesh screen. Forinstance, highly satisfactory results are obtained from compositionswherein the grains are retained on an 18-mesh screen.

In its most preferred form our invention may be described as a granularpowder displaying an apparent density measured by a stick count of from150 to 215 cartridges of 1%" x 8" in size for 50 lbs. of powder. Thissame stick count range holds for 1 /2 x 6" cartridges. Our preferredcompositions display a sensitiveness of at least 10" in the halfcartridge sensitiveness test, and exhibit velocities below 1450 metersper sec-- ond. These characteristics are preferred features and are notto be considered as limiting the broad scope of the invention.

In greater detail, we blend a composition comprising amonium nitratewith a material capable of exerting a wetting action on the surface ofthe ammonium nitrate particles and other materials present under theinfluence of a small amount of aqueous solution or other suitableliquid, and of becoming a tenacious solid subsequently, thus functioningas an adhesive to bind the ingredients together. The blended material soconstituted is in the form of firm, free-flowing grains of controlledsize, and these grains are given a coating of liquid explosive nitricester in amount sufficient to bring to detonation the entire explosivecomposition, yet not exceeding 10% by weight.

As binding agent or adhesive, we find starch highly efiective andadvantageous, since, in the presence of water or aqueous solution andunder the influence of heat, the starch assumes a gelatinous condition.On subsequent cooling or removal of solvent, the starch becomes hard andtenacious, exerting a firm adhesive effect upon the materials which ithas previously contacted. We do not intend to be limited to the use ofstarch as binding agent, however, and various other materials can becaused to function satisfactorily, for example, agar-agar; casein,animal glue, and the like; in fact, any material capable of becomingmucilaginous and adhering to the solid ingredients present, and ofbecoming hard and tenacious on cooling or loss of solvent. As alreadynoted, the binding agent must be substantially insoluble innitroglycerin or other explosive nitric esters in order to be utilizablesatisfactorily according to our invention.

Our invention is particularly applicable to explosives high in ammoniumnitrate content, for example, between and The available oxygen presentin ammonium nitrate requires the 'all of the ingredients.

presence of sufiicient oxygen-accepting material, if an effectiveoxygen-balanced explosive is to result. Starch itself or other organicadhesive may serve this purpose, and We find excellent results to beobtained when an essentially starchy rial presumably become filled withthe adhesive or the material becomes completely coated therewith.

In order to bring about gelatinization of the adhesive, it is sufficientto wet the surface thereof. This can be accomplished by the directintroduction of water into the mixture of some or The presence of anexcess of Water would defeat the purpose of its use, and we find itdesirable to introduce an amount of water not exceeding 5% of theexplosive composition, for example, between 1.5 and 3.5%. With such awater content, the starch becomes of mucilaginous consistency onheating, and brings the mixture into plastic form. On cooling, theadhesive becomes hard and tenacious and serves as an effective bindingagent.

While the use of water has been described for bringing aboutgelatinization of the adhesive, other liquids may serve also. We may,for example, employ substantially anhydrous ammonia for this purpose,and in this case heating may be unnecessary, since gelatinization takesplace in the cold and the solvent can be readily removed byvolatilization. A very effective method also of introducing liquid intothe mixture of ingredients is by the employment of salts containingwater of crystallization. Hydrated sodium thiosulfate (Na2S2O3-5H2O),sodium acetate (NaC2HaO2-3H2O) sodium tetraborate and the like areexamples of such salts, and we find sodium thiosulfate particularlyeffective for this purpose. In using salts containing water ofcrystallization, the blend with ammonium nitrate and potential adhesiveis heated until the salt melts in its own water of crystallization.Fusion of ammonium nitrate is thereby facili tated, and the liquidpresent serves to bring about gelatinization of the adhesive. In usingsalts containing water of crystallization, the water content isdesirably controlled in substantially the amount previously stated to bemost effective, namely, not over 5% of the entire composition. In placeof hydrated salts, we may likewise employ' anhydrous salts or saltsserving merely as agents to facilitate the fusion of ammonium nitrate,with or without the addition of water to the blend.

The principle of our invention is to blend together the materialsdescribed in the foregoing, form them into granular agglomerates of acontrolled size, and then efiect proper sensitization. In mixing theingredients with the addition of heat, a plastic blend is attained.While still in the plastic state, the material is granulated, preferablyby passing through a screen of the desired mesh. 0n cooling, firm, hardgrains result and the finished explosive is obtained by introducingsufiicient nitroglycerin or other liquid explosive nitric ester to givethe desired surface coating.

The use of starch or other nitroglycerin-insoluble adhesive insures thatthere will be little penetration of the liquid explosive within thegrains; hence, increased sensitiveness results with a minimum of liquidexplosive. As noted, we find it desirable to use not more than 10% ofnitroglycerin, and a favorable amount is between 5 and 8%.

The following will serve as examples of the methods of carrying out ourinvention and the product obtained thereby.

Example 1 3950 grams of ammonium nitrate, of such a degree of finenessthat not more than 20% was held on a 35-mesh screen, was introduced intoa mixing apparatus, together with 250 grams of sodium nitrate and 25grams of calcium carbonate. After agitation had begun, the dry saltmixture was heated to 75 C. by means of steam in the surrounding jacket.305 grams of hydrated sodium thiosulfate was then added, followedimmediately by 150 grams of starch. Mixing was continued until thestarch had become gelatinized and the charge was of such consistencythat it resembled bread dough, and remained as a single mass adhering tothe revolving mixer blades. 320 grams of bagasse pulp was then added andmixed thoroughly into the blend. The temperature was maintained below C.After addition of the bagasse, the mixture was cooled by means of coldwater introduced into the jacket. When the charge had partially cooledbut while it was still plastic, it was discharged from the mixer andpassed through a 6-mesh screen. After cooling to room temperature, thegrains were tough and durable. The material held on a 30-mesh screen wasmixed with nitroglycerin in the proportion of 93.5 parts of blend to 6.5parts of nitroglycerin. When packed in 1 x 8" cartridges, 167 cartridgesper 50 lbs. were obtained. The explosive had a velocity of detonation of1345 m./sec., and a sensitiveness of 12", as obtained by the propagationtest, where two half-sticks of the explosive are separated from oneanother and the maximum distance of separation is determined at whichconsistent detonation of the second cartridge by the first takes place.

Example 2 3850 grams of relatively fine ammonium nitrate was introducedinto the mixer, along with 500 grams of finely divided sodium nitrateand 25 grams of chalk. The dry mixture was agitated and the temperatureraised to around 80 C. by means of steam. 500 grams of a flaked cornproduct was then added and grams of water. The mixing was continueduntil the starch had become gelatinized and the proper consistencyobtained. The mix was then cooled, screened, and sensitized as inExample 1, 6.5 parts of nitroglycerin being used per 93.5 parts of thesalt blend. The finished explosive had a velocity of 1380 m./sec. and asensitiveness of 10". In pre paring the explosive of this example, theflaked corn product used was of the approximate size of granule desiredin the sensitized explosive product.

Example 3 3825 grams of ammonium nitrate of relatively fine granulationwas introduced into the mixer,

together with 500 grams of sodium nitrate and 25 grams of chalk. Afteragitation had begun, the temperature was raised to about C., and 400grams of sawdust, 150 grams of starch and I00 grams of water were added.Mixing was continued until the desired gelatinous condition had beenobtained on the starch, and the charge had acquired the properconsistency. After partial cooling, the mixture was screened as inExample 1, and the hard grains were sensitized by the addition ofnitroglycerin in the amount of 6.5 parts to 93.5 parts of the solidmixture. The cartridged explosive had a velocity of 1316 m./sec. and asensitiveness of 10".

Example 4 3875 grams of ammonium nitrate of fine granulation wasintroduced into the mixer, together with 250 grams of sodium nitrate and25 grams of calcium carbonate. The mixture wasagitat- 'ed, thetemperature was raised to 80 C., and

Example 5 3925 grams of ammonium nitrate, 250 grams of sodium nitrateand 25 grams of chalk were introduced into the mixing apparatus. Duringagitation, the temperature of the ingredients was raised by means ofsteam in the jacket to approximately C. 300 grams of hydrated sodiumthiosulfate and 500 grams of a flaked corn product were then added.Mixing was continued until the starch of the flaked corn product hadbecome gelatinized and the proper consistency had been obtained in thecharge. The mixture was then cooled somewhat and screened as inExample 1. Nitroglycerin was incorporated with the firm, granularmaterial in an amount corresponding to 6.5 parts nitroglycerin to 93.5parts of the grains. The cartridged explosive had a velocity of 1320meters per second and a sensitiveness of 12 to 15".

From the procedure described in the examples above, and from thediscussion of our invention in the foregoing, it will be understood thatthe invention has to do with explosives high in ammonium nitrate andthat various other salts may be present with the ammonium nitrate. Suchother salts may include various oxidizing agents, particularly sodiumnitrate, though other similar oxygen-carrying salts may be used equallywell, for example, potassium nitrate, magnesium nitrate, calciumnitrate, and others. It may be desirable also to use various materialsas fusion promoters for the ammonium nitrate, for exam ple, the varioushydrated salts, urea, and other materialssoluble in molten ammoniumnitrate. Such saltsand compounds, by promoting the fusion of ammoniumnitrate, assist in bringing about the wetting of the surface of theadhesive.

In coal blasting ammonium nitrate explosives of the prior art, it hasbeen the almost invariable custom to employ ammonium nitrate ofrelatively coarse granulation, since the control of particle size ofthis explosive salt has been the recognized method for varyingdetonation velocity. We find, however, that the use of coarse ammoniumnitrate is not necessary for making the granular explosive of ourinvention; Instead we find that the use of relatively fine ammoniumnitrate is quite satisfactory for this purpose. For instance, suitablepowders according to our invention are prepared with ammonium nitrate ofsuch grain size that at least 40% will pass a 48-mesh screen, while notmore than 10% will be held on a 24-mesh screen. This range issatisfactory but not limiting.

As has been stated, our finished explosive is in the form of relativelycoarse grains which are firm and free-flowing. While a range of grainsize can be employed, we find it preferable that substantially all beheld on a 30-mesh screen. In the examples cited, the preferred method ofpreparing the grains was to screen the blended material while still inplastic condition. Other methods for preparing the grains may beemployed, however, such as passing the plastic material into anextruderoperated on the meatchopper principle and extruding through a dischaving openings of predetermined size. The method of preparing thegrains is not a part of our invention.

It should be emphasized that the material used as adhesive for obtaininga firm, rigid grain must be one which is substantially insoluble innitroglycerin or other liquid explosive nitric ester. In

this way penetration of the liquid explosive into the grain isprevented, and increased sensitiveness thereby results.

The advantage of the product according to our invention lies in the factthat a permissible explosive is obtained of highly attractive propertiesas a lump coal-getter. In spite of the low velocity, there is nosacrifice in strength or exe cution. To an unusual degree the slowheaving action of black powder is obtained, with all the safety featuresof permissible explosives high-in ammonium nitrate. Due to the increasedsensitizing action of a given amount of nitroglycerin in the explosivecomposition, a low nitroglycerin content is sufiicient to obtain thehigh degree of sensitiveness previously brought atbout only by the useof objectionably large amounts of nitroglycerin.

Furthermore, our product possesses marked superiority over explosivecompositions of the pulverulent type having approximately the samecomposition. For instance, comparisons between our granular powdersand-pulverulent powders having identical low nitroglycerin contentsclear- 1y establish that our particular granular compositions displayincreased propagating ability, lower velocities and .improved heavingaction in coal blasting.

While our invention has been described in detail in the foregoing, itwill be understood that many variations may be made in the exact methodof preparation or in compositions without departure from the spirit ofthe invention. We intend to be limited. therefore, only by the followingpatent claims.

We claim:

1. The method of preparing a granulated explosive, which methodcomprises intimately intermingling ammonium nitrate and a material whichis substantially insoluble in liquid explosive nitric esters and whichis capable of functioning as a tenacious adhesive when subjected to theaction of a wetting liquid and subsequently solidified, introducing arelatively small amount of a wetting liquid into contact with the mixedingredients, forming the blend of materials, while still plastic, intodistinct agglomerates of controlled size, cooling to convert saidagglomerates into firm, free-flowing grains, and coating the surface ofsaid grains with a liquid explosive nitric ester.

2. The method of preparing a granulated explosive, which methodcomprises intimately intermingling ammonium nitrate and a material whichis substantially insoluble in liquid explosive nitric esters and whichis capable of functioning as a tenacious adhesive when subjected to theaction of heat and an aqueous liquid and subsequently solidified, addinga small amount of an aqueous liquid to the mixture and subjecting theblended mixture to the action of heat, forming the blend of materials,while still plastic, into distinct agglomerates of controlled size,cooling to convert said agglomerates into firm, freeflowing grains, andcoating the surface of said grains with nitroglycerine in an amount lessthan 10% of the entire composition.

3. The method of preparing a granulated explosive, which methodcomprises intimately intermingling ammonium nitrate and starch,introducing into the mixture water in an amount less than 5% of theentire composition, subjecting the mixture to the action of heat,forming the blend of materials, while still plastic, into distinctagglomerates of controlled size, cooling to convert said agglomeratesinto firm, free-flowing grains, and coating the surface of said grainswith nitroglycerin in an amount less than 10% of the entire composition.

4. The method of preparing a granulated explosive, which methodcomprises intimately intermingling a preponderant amount of ammoniumnitrate, starch, a carbonaceous combustible ingredient, and water in anamount less than 5% of the entire composition, subjecting the mixture tothe action of heat, forming the blend of materials, while still plastic,into distinct agglomerates of controlled size, cooling to convert saidagglomerates into firm, free-flowing grains, and surface-coating saidgrains with nitroglycerin in an amount less than 10% of the entirecomposition.

5. The method of preparing a granular explosive, which method comprisesintimately intermingling a preponderant amount of ammonium nitrate,starch, a carbonaceous ingredient, and a salt containing water ofcrystallization, subjecting the blended mixture to a temperaturesufilcient to fuse said salt containing water of crystallization,forming the blend of materials into distinct agglomerates of controlledsize, cooling to convert said agglomerates into firm, freefiowinggrains, and coating the surface of said grains with nitroglycerin in anamount less than 10% of the entire composition.

6. The method of preparing a granular explosive, which method comprisesintimately intermingling ammonium nitrate and starch, in-

troducing into the mixture sufficient substantially anhydrous liquidammonia to effect a gelatinous condition on said starch, forming theblend of materials, while still plastic, into distinct agglomerates ofcontrolled size, cooling to convert said agglomerates into firm,free-flowing grains, and surface-coating said grains with an amount ofnitroglycerin sufficient to bring about detonation of the entirecomposition.

MELVIN A. COOK. DONALD B. GAWTHROP. MILTON H. WAHL. CLIFFORD A.WOODBURY.

